CVE-2026-66035Network attack vector

Pre-authentication heap corruption via undersized ETM packets

Published Jul 24, 2026 · Updated Jul 24, 2026

Heap buffer overflow in libssh2 through 1.11.1 allows remote attackers controlling an SSH server to corrupt connecting clients. The transport_fullpacket function allocates decrypt_buffer from the packet-derived decrypt size, then copies a cipher block without first rejecting an ETM packet shorter than that block. A client must initiate the SSH session and negotiate Encrypt-then-MAC; successful exploitation can overlap heap objects and overwrite a function pointer before authentication.

CVSS severity7.7
High
EPSS probability0.47%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap buffer overflow in libssh2 through 1.11.1 allows remote attackers controlling an SSH server to corrupt connecting clients. The transport_fullpacket function allocates decrypt_buffer from the packet-derived decrypt size, then copies a cipher block without first rejecting an ETM packet shorter than that block. A client must initiate the SSH session and negotiate Encrypt-then-MAC; successful exploitation can overlap heap objects and overwrite a function pointer before authentication.

The record

CVE
CVE-2026-66035
Published
Jul 24, 2026
Updated
Jul 24, 2026
Vendor
libssh2
Product
libssh2
Classifications
CWE-122, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <=1.11.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

libssh2 · libssh2version=0 <=1.11.1

Published CVSS scores

7.7VulnCheckCVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
7.5VulnCheckCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-122Heap-based Buffer Overflow
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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